Recombinase Polymerase Amplification (RPA) is a process in which recombinase-mediated targeting of oligonucleotides to DNA targets is coupled to DNA synthesis by a polymerase (U.S. Pat. No. 7,270,981 filed Feb. 21, 2003; U.S. Pat. No. 7,399,590 filed Sep. 1, 2004; U.S. Pat. No. 7,435,561 filed Jul. 25, 2006 and U.S. Pat. No. 7,485,428 filed Aug. 13, 2007, as well as, U.S. application Ser. No. 11/628,179, filed Aug. 30, 2007; Ser. No. 11/800,318 filed May 4, 2007 and 61/179,793 filed May 20, 2009; the disclosures of the foregoing patents and patent applications are each hereby incorporated by reference in its entirety). RPA depends upon components of the cellular DNA replication and repair machinery. The notion of employing some of this machinery for in vitro DNA amplification has existed for some time (Zarling et al., U.S. Pat. No. 5,223,414), however the concept has not transformed to a working technology until recently as, despite a long history of research in the area of recombinase function involving principally the E. coli RecA protein, in vitro conditions permitting sensitive amplification of DNA have only recently been determined (Piepenburg et al. U.S. Pat. No. 7,399,590, also Piepenburg et al., PlosBiology 2006). Development of a ‘dynamic’ recombination environment having adequate rates of both recombinase loading and unloading that maintains high levels of recombination activity for over an hour in the presence of polymerase activity proved technically challenging and needed specific crowding agents, notably PEG molecules of high molecular weight (e.g., Carbowax 20M molecular weight 15-20,000 and PEG molecular weight 35,000), in combination with the use of recombinase-loading factors, specific strand-displacing polymerases and a robust energy regeneration system.
The RPA technology depended critically on the empirical finding that high molecular weight polyethylene glycol species (particularly >10,000 Daltons or more) very profoundly influenced the reaction behavior. It has previously been discovered that polyethylene glycol species ranging in size from at least molecular weight 12,000 to 100,000 stimulate RPA reactions strongly. While it is unclear how crowding agents influence processes within an amplification reaction, a large variety of biochemical consequences are attributed to crowding agents and are probably key to their influence on RPA reactions.
Crowding agents have been reported to enhance the interaction of polymerase enzymes with DNA (Zimmerman and Harrison, 1987), to improve the activity of polymerases (Chan E. W. et al., 1980), to influence the kinetics of RecA binding to DNA in the presence of SSB (Lavery P E, Kowalczykowski S C. J Biol Chem. 1992 May 5; 267(13):9307-14). Crowding agents are reported to have marked influence on systems in which co-operative binding of monomers is known to occur such as during rod and filament formation (Rivas et al., 2003) by increasing association constants by potentially several orders of magnitude (see Minton, 2001). In the RPA system multiple components rely on co-operative binding to nucleic acids, including the formation of SSB filaments, recombinase filaments, and possibly the condensation of loading agents such as UvsY. Crowding agents are also well known to enhance the hybridization of nucleic acids (Amasino, 1986), and this is a process that is also necessary within RPA reactions. Finally, and not least, PEG is known to drive the condensation of DNA molecules in which they change from elongated structures to compact globular or toroidal forms, thus mimicking structures more common in many in vivo contexts (see Lerman, 1971; also see Vasilevskaya. et. al., 1995; also see Zinchenko and Anatoly, 2005) and also to affect the supercoiling free energy of DNA (Naimushin et al., 2001).
Without intending to be bound by theory, it is likely that crowding agents influence the kinetics of multiple protein-protein, protein-nucleic acid, and nucleic acid-nucleic acid interactions within the reaction. The dependence on large molecular weight crowding agents for the most substantial reaction improvement (probably greater than about 10,000 Daltons in size) may reflect a need to restrict the crowding effect to reaction components over a certain size (for example oligonucleotides, oligonucleotide:protein filaments, duplex products, protein components) while permitting efficient diffusion of others (say nucleotides, smaller peptides such as UvsY). Further, it may also be that the high molecular weight preference might reflect findings elsewhere that as PEG molecular weight increases the concentration of metal ions required to promote DNA condensation decreases. In any case it is an empirical finding that RPA is made effective by the use of high molecular weight polyethylene glycols.
In addition to a need for specific type of ‘crowded’ reaction conditions as described above (reaction in the presence of crowding agents), effective RPA reaction kinetics depend on a high degree of ‘dynamic’ activity within the reaction with respect to recombinase-DNA interactions. In other words, the available data which includes (i) reaction inhibition by ATP-γ-S, or removal of the acidic C terminus of RecA or UvsX, and (ii) inhibition by excessive ATP (Piepenburg et al., 2006) suggest that not only is it important that recombinase filaments can be formed rapidly, but also important that they can disassemble quickly. This data is consistent with predictions made in earlier U.S. Pat. No. 7,270,981. Rapid filament formation ensures that at any given moment there will be a high steady state level of functional recombinase-DNA filaments, while rapid disassembly ensures that completed strand exchange complexes can be accessed by polymerases.